Temperature control for low temperature reduction cell

ABSTRACT

An improved method of producing aluminum in an electrolytic cell containing alumina dissolved in an electrolyte, the method comprising the steps of providing a molten salt electrolyte at a temperature less than 900° C. having alumina dissolved therein in an electrolytic cell having a liner for containing the electrolyte, the liner having a bottom and walls extending upwardly from the bottom, the liner being substantially inert with respect to the molten electrolyte. A plurality of non-consumable anodes and cathodes are disposed in the electrolyte and an electric current is passed through the anodes and through the electrolyte to the cathodes depositing aluminum on the cathodes and generating oxygen bubbles at the anodes, the bubbles stirring the electrolyte. Periodically, the electric current flow to the cell is reduced for extended periods. The electrolyte and aluminum in the cell is maintained in a molten condition during the extended periods of reduced current flow by application of heat to the bottom for purposes of heating the cell.

BACKGROUND OF THE INVENTION

[0001] This invention relates to aluminum reduction cells and moreparticularly, it relates to a method for controlling the temperature inlow temperature electrolytic reduction cells used for production ofaluminum from alumina dissolved in a molten salt electrolyte.

[0002] The use of low temperature (less than about 900° C.) electrolyticcells for producing aluminum from alumina have great appeal because theyare less corrosive to cermet or metal anodes and other materialscomprising the cell. The Hall-Heroult process, by comparison, operatesat temperatures of about 950° C. This results in higher aluminasolubility but also results in greater corrosion problems. Also, in theHall-Heroult process, carbon anodes are consumed during the process andmust be replaced on a regular basis. In the low temperature cells,non-consumable anodes are used and such anodes evolve oxygen instead ofcarbon dioxide which is produced by the carbon anodes.

[0003] However, operation of low temperature electrolytic cells is notwithout problems. For example, before operating the cell, there is theneed to heat the cell to operating temperature. In a conventionalHall-type cell, the temperature of the cell is raised at startup byelectrical resistance heating of a coke bed in the cell cavity or bydirecting gas flames into the cell cavity. It will be appreciated thatthese methods of heating are not suitable for low temperature cells.

[0004] Another problem with low temperature cells includes controllingtemperature of the cell during operation. In conventional cells,temperature of operation can be controlled by changing the distancebetween the anode and the cathode. However, this method is notapplicable in a low temperature cell employing fixed inert anodes anddimensionally stable cathodes because the anode-cathode distance isusually set at startup. U.S. Pat. Nos. 5,415,742 and 5,279,715 suggestvarying the power input to an electrolytic cell to control temperatureby varying the amount of electrolyte between the electrodes or varyingthe extent of cross-sectional area available for current flow betweenthe anodes and the cathodes.

[0005] U.S. Pat. No. 4,333,803 discloses a method and apparatus formaintaining a predetermined energy balance in a device, such as analuminum reduction cell. The apparatus includes a relatively short andthin heat flow sensor having a first and second thermocouple locatedwithin opposite closed ends of a hollow thermally conductive body. Eachthermocouple is composed of two wires of the same dissimilar metals. Thesensor is secured by one closed end of the sensor body to an outsidesurface of the wall member to extend substantially perpendicular to thelocation on the wall without significantly affecting the heat flow fromthe wall surface being measured.

[0006] U.S. Pat. No. 5,882,499 discloses a process for regulating thetemperature of electrolytic cells. It involves acting on the temperatureof the pot by means of the setpoint resistance Ro which is modulated soas to correct the temperature both by anticipation and by reversedfeedback. Correction by anticipation, known as “a priori” correction,allows for known, quantified disturbances and allows their effect on thetemperature of the pot to be compensated in advance. Reversed feedbackcorrection, known as “a posteriori” correction, involves determining,from direct measurement at regular time intervals of the temperature ofthe electrolytic bath, a mean temperature corrected as a function ofperiodic operating procedures and allows the variations and deviationsfrom the setpoint temperature to be compensated.

[0007] U.S. Pat. No. 466,460 discloses recovering aluminum from aluminumchloride by electrolysis when the aluminum chloride is heated to a hightemperature and pressure.

[0008] U.S. Pat. No. 473,866 discloses employing an electric current toeffect electrolytic decomposition, and maintaining the state of fusionby the combined heating effects of such current and a flame or likeauxiliary source of heat which, like the heat due to the current, actsdirectly on the ore next the electrodes rather than through the walls ofa furnace or crucible.

[0009] U.S. Pat. No. 3,632,488 discloses a method of controlling analuminum reduction cell in which the heat flow coefficient for the bathis determined and used with a desired bath temperature to calculate thebath's heat loss energy. Calculations are also made of the cell's powerrequirements for purposes other than heating the bath such as the energyrequired to reduce the cell's alumina. The sum of the bath heat loss andother energy requirements is divided by the cell's base amperage todetermine a set voltage and the cell's anode is adjusted to keep thecell voltage within predetermined limits of the set voltage.

[0010] U.S. Pat. No. 4,045,309 discloses that the energy balance in analuminum reduction cell is controlled by measuring the temperature ofthe side lining of the cell, preferably at the level of the surface ofthe electrolyte, comparing the measured temperature with a referencetemperature, and when the difference between the measured and referencetemperatures exceeds a given value, adjusting the depth of immersion ofthe cell anodes within the electrolyte.

[0011] U.S. Pat. No. 4,146,444 discloses a method for preheating amolten salt electrolysis cell having an electrode which includes atleast one element protruding into the interior of the cell. The methoddisclosed includes the distribution of a carbonaceous aggregate aroundsuch an element, and the ignition of this aggregate, so that the elementmay be brought to an elevated temperature without breaking due to theeffects of thermal gradients.

[0012] U.S. Pat. No. 4,181,584 discloses a method for heating anelectrolytic cell wherein holes are drilled in the solid electrolyte,e.g., to the floor of the cell, to provide space for supporting blocks.The holes are spaced a predetermined distance apart to position at leastone anode between them. Supporting blocks with a length sufficient toextend from the floor to at least the level of the solid electrolytebeneath the anode are placed in the holes and a resistance heater,preferably one having a positive change in resistivity with temperature,is disposed between the supporting blocks at least one of which iselectrically conductive. The anode of the cell is lowered intoelectrical contact with the resistance heater and current sufficient toheat the resistance heater to at least the melting temperature of theelectrolyte is passed from the anode through the resistance heater.Heating of the resistance heater is continued until the solidelectrolyte in the cell has melted.

[0013] U.S. Pat. No. 4,608,135 relates to an improvement in a Hall cell.The improvement comprises an air passageway between the insulating layerand the outer surface of the carbon lining sidewall and an air inletport adjacent the bottom of the passageway for passing air into the airpassageway and along the outer surface of the carbon lining sidewallwhereby the carbon sidewall may be cooled sufficiently to permit theformation of a protective layer of frozen bath on the inner surfacethereof. The heated air then flows across the top of the cell wherebythe cell retains at least a part of the heat exchanged through thesidewall.

[0014] U.S. Pat. No. 4,865,701 discloses that alumina is reduced tomolten aluminum in an electrolytic cell containing a molten electrolytebath composed of halide salts and having a density less than alumina andaluminum and a melting point less than aluminum. The cell comprises aplurality of vertically disposed, spaced-apart, non-consumable,dimensionally stable anodes and cathodes. Alumina particles aredispersed in the bath to form a slurry. Current is passed between theelectrodes, and oxygen bubbles form at the cathodes. The oxygen bubblesagitate the bath and enhance dissolution of the alumina adjacent theanodes and inhibit the alumina particles from settling at the bottom ofthe bath. The molten aluminum droplets flow downwardly along thecathodes and accumulate at the bottom of the bath.

[0015] In spite of these disclosures, there is still a great need for animproved low temperature electrolytic cell for producing aluminum fromalumina which includes means for bringing the cell to operatingtemperature, a method of controlling the temperature of the cell duringoperation and means for maintaining the cell at temperature whilereducing the electric current input for load leveling purposes duringpeak power demand periods.

SUMMARY OF THE INVENTION

[0016] It is an object of the invention to provide a method forproducing molten aluminum in an electrolytic cell.

[0017] It is another object of the invention to provide an improvedmethod and apparatus for raising the temperature of a low temperature,electrolytic cell for the production of aluminum to operatingtemperature.

[0018] It is another object of the invention to provide a method forcontrolling the temperature of a low temperature electrolytic cellduring the production of aluminum from alumina.

[0019] It is still another object of the invention to provide a methodand apparatus for maintaining a low temperature cell used for productionof aluminum at temperature during periods of reduced electric currentinput to the cell during peak power demand.

[0020] These and other objects will become apparent from a reading ofthe specification and claims appended hereto.

[0021] In accordance with these objects, there is provided an improvedmethod of producing aluminum in an electrolytic cell containing aluminadissolved in an electrolyte, the method comprising the steps ofproviding a molten salt electrolyte at a temperature less than 900° C.having alumina dissolved therein in an electrolytic cell having a linerfor containing the electrolyte, the liner having a bottom and wallsextending upwardly from the bottom, the liner being substantially inertwith respect to the molten electrolyte. A plurality of non-consumableanodes and cathodes are disposed in the electrolyte and an electriccurrent is passed through the anodes and through the electrolyte to thecathodes depositing aluminum on the cathodes and generating oxygenbubbles at the anodes, the bubbles stirring the electrolyte.Periodically, the electric current flow to the cell is reduced forextended periods. The electrolyte and aluminum in the cell is maintainedin a molten condition during the extended periods of reduced currentflow by application of heat to the bottom for purposes of heating thecell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a plan view illustrating an embodiment of the inventionwhich may be used in the practice of the invention.

[0023]FIG. 2 is a cross-sectional view of an electrolytic cell alongline A-A of FIG. 1.

[0024]FIG. 3 is a cross-sectional view of an electrolytic cell alongline B-B of FIG. 1.

[0025]FIGS. 4A and 4B are cross-sectional views of a channel used fordelivering molten aluminum.

[0026]FIG. 5 is a cross-sectional view of an electrolytic test cellshowing a conduit or collector in connection with cathodes fordelivering molten metal to a reservoir.

[0027]FIG. 6 is a view along line C-C of FIG. 5.

[0028]FIG. 7 is a cross-sectional view along line D-D of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] In FIG. 1, there is shown a top or plan view of an embodiment ofthe invention which illustrates an electrolytic cell 2 for theelectrolytic production of aluminum from alumina dissolved in anelectrolyte contained in the cell. Cell 2 comprises a metal or alloyliner 4 having bottom and sides for containing electrolyte.Non-consumable or inert anode 6 is shown mounted vertically inside liner4 which preferably has the same composition as anode 6. Further, asshown in FIG. 1, anode 6 is connected to liner 4 by means of straps 8 toprovide an electrical connection therebetween. Also, liner 4 is shownconnected to bus bar 14A by electrical conducting strap 9. Cathodes 10are shown positioned on either side of anode 6. Cathodes 10 areelectrically connected to bus bar 14B by appropriate connection meanssuch as strap 16. Liner 4 is layered with thermal insulating material 18such as insulating fire brick which is contained with a metal shell 20.

[0030] In operation, electrical current from bus bar 14A flows throughelectrical strap 9 into anodic liner 4. Current from liner 4 flowsthrough conducting straps 8 to anodes 6 then through an electrolyte tocathodes 10. The current then flows from cathodes 10 along connectionmeans 16 to a second bus bar 14B. Additional electrolytic cells may beconnected in series on each side of cell 2.

[0031] While any inert anode including cermets or metal alloys may beused in the electrolytic cell of the invention, it is preferred that theanode material including the anodic liner be comprised of Cu—Ni—Fecompositions that have resistance to oxidation by the electrolyte.Suitable anode compositions are comprised of 25-70 wt. % Cu, 15-60 wt. %Ni and 1-30 wt. % Fe. Within this composition, a preferred anodecomposition is comprised of 35-70 wt. % Cu, 25-48 wt. % Ni and 2-17 wt.% Fe with typical compositions comprising 45-70 wt. % Cu, 28-42 wt. % Niand 13-17 wt. % Fe.

[0032] It will be noted that a number of anodes and cathodes is employedwith the anodes and cathodes used in alternating relationship.

[0033] In the plan view in FIG. 1, there is shown a schematic of conduit30 (see also FIGS. 2 and 3) for conveying molten aluminum from cathodes10 to a molten aluminum reservoir 34. In FIG. 1, molten aluminumreservoir 34 is shown contained within liner 4. Thus, aluminum producedat cathodes 10 is collected in conduit 30 and is conveyed to moltenaluminum reservoir 34 for removal from the cell.

[0034]FIG. 2 is a cross-sectional view along line A-A of FIG. 1 showinganodic liner 4, straps 8 connecting anodes to the liner, cathode 10,strap 9 connecting liner 4 to bus bar 14A and insulation 18 containedbetween anodic liner 4 and metal shell 20. Also, shown in FIG. 2 iselectrical connection means 16 used to connect cathodes 10 to bus bar14B. Connection means 16 may be comprised of a flexible metal strap 22which is connected to bus bar 14B. Flexible metal strap 22 is connectedto cathode 10 by collector bars 24 which are slotted on the bottom andstraddle or fit over cathode 10. Strap 22 is connected to collector bar24 utilizing an aluminum cap 26. That is, aluminum cap 26 is cast oncollector bar 24 and strap 22 is welded thereto. Electrical connectionbetween the cathode and collector bar may be provided by using aluminummetal at the connection. That is, aluminum metal becomes molten at celloperating temperature and wets both the cathode and collector bar,particularly if both cathode and collector bar are fabricated fromtitanium diboride. To guard against air burn of collector bar 24 duringoperation, a sleeve or tube of alumina 28 or like material may be usedto cover or surround collector bar 24.

[0035] Referring further to FIG. 2, it will be seen that anodic liner 4has vertical sides 32 and bottom referred to generally as 36. Bottom 36has two sides 38 which are contiguous with walls or sides 32. Sides 38of bottom 36 are sloped downwardly towards a central trough or channel40. Channel 40 is filled with an electrical insulating material 42,substantially non-reactive with bath or aluminum. Electrical insulatingmaterial 42 may be selected from alumina and boron nitride or othersuitable non-reactive material. A tube 44 of refractory material, e.g.,titanium diboride, is positioned in insulating material 42 to carrymolten aluminum away from cathodes 10 to reservoir 34.

[0036] Cathodes 10 are shown positioned under surface 46 of electrolyte45 and spaced substantially equally from sides 32 of liner 4. Cathodes10 have a lower surface or edge 48 which rest on blocks 50 which areelectrically insulating blocks, e.g., alumina or boron nitride blocks.Lower surface or edges 48 are shown positioned parallel to sides 38 ofliner 4. Cathodes 10 terminate in a point or end 52 provided in slottedopening 58 in tube 44 (see FIG. 3). In operation of the cell, aluminumdeposited on the cathode flows towards point or end 52 and into tube 44from where it is removed to reservoir 34. Grooves 54 may be provided incathode 10 to aid in the flow of molten aluminum on the cathode surfacetowards point or end 52 for purposes of collection.

[0037]FIG. 3 is a cross-sectional view along line B-B of FIG. 1 showingliner 4, anodes 6, cathodes 10, molten aluminum reservoir 34, andrefractory tube 44 for transferring or carrying molten aluminum fromcathodes 10 to molten aluminum reservoir 34. It will be noted thatrefractory tube 44 has a central bore 56 having slotted openings 58therein approximate or adjacent cathodes 10. Openings 58 permit moltenaluminum collected at the cathodes to pass into bore 56 and flow towardsmolten aluminum reservoir 34. Molten aluminum in bore 56 passes throughopening 60 into molten aluminum reservoir 34 where a body 62 of moltenaluminum collects therein. A layer 64 of electrolyte 45 may be providedon top of body 62 to protect against oxidation of molten aluminum withair. The head of electrolyte or bath contained by liner 4 forcesaluminum from the cathodes into bore 56 and therefrom into reservoir 34.The aluminum produced is collected continuously from all the cathodesand directed to body 62 which is contained in an electrically insulatedvessel or reservoir.

[0038] While not wishing to be bound by any theory of invention, thecollection of body 62 of aluminum is explained as follows. That is, withreference to FIG. 3, there is shown the heads of electrolyte in cell 2.Also shown is the head of aluminum reservoir 34. The top of tube 44 isused as the reference plane. The head of electrolyte in cell 2 isdenoted as h_(b1) and the total head in collection vessel or reservoir34 is denoted as h_(a)+h_(b2). The pressure from the headsh_(a)+h_(b2)must be less than the pressure from the electrolyte or bathhead h_(b1) to prevent aluminum leaking out of joints or openings 58between cathodes 10 and tube 44. This concept may be represented by thefollowing formula:

h _(b1)ρ_(b1) ≧h _(a)ρ_(a) +h _(b2)ρ_(b2)  Eq.(1)

[0039] If equality is used in Eq.(1) and the following values areassumed,

[0040] h_(b1)=45 cm (i.e., 18 inch high cathodes)

[0041] h_(b2)=5 cm

[0042] ρ_(b1)=1.97 g/cm³

[0043] ρ_(b2)=1.97 g/cm³

[0044] ρ_(a)=2.36 g/cm³

[0045] these values give h_(a) (max.)=33 cm, or a total maximum head(h_(b2)h_(a)) in the collection vessel of 38 cm.

[0046] Aluminum 62 is removed from reservoir 34 by periodic siphoning.When the aluminum is tapped from collection vessel 34, the headdifference between the bath and the vessel is 45-5=40 cm. Bath then hasto be excluded from tube 44 by interfacial tension of aluminum/bath inslots or openings 58 between the cathodes 10 and tube 44. The width ofslot or opening 44 can be calculated by:

t≦2γ/Δhρg, where t is the width of opening 58  Eq.(2)

[0047] Using the following values:

[0048] γ=500 dyne/cm

[0049] Δh=40 cm

[0050] ρ=1.97 g/cm³

[0051] g=980 dyne/gm

[0052] gives t (max.)=0.013 cm (0.13 mm or 130 μm).

[0053] Thus, for a cell of this size, the width of opening 58 would havebe on the order of 130 μm.

[0054] During startup of a cell, there is a substantial increase intemperature. Thus, it may be necessary to accommodate the differentialexpansion between lining 4 and refractory tube 44. FIGS. 4A and 4Billustrates joints which may be used in conjunction with refractory tube44. These joints permit differential expansion between lining 4 andrefractory tube 44 during cell startup. It will be seen from FIG. 4Athat refractory tube 44 is comprised of joints 68 where the one end oftube 44 fits into another part of tube 44. A space is provided at joint68 to care for any differential expansion which may occur between lining4 and refractory tube 44. In FIG. 4B, another type of joint is disclosedto accommodate differential expansion during startup of cell 2. That is,at joint 70, a tubular member 72 is provided inside refractory tube 44overlapping joint 70 to ensure against leakage and yet provide fordifferential thermal expansion. Tubular member 72 may be comprised ofthe same or similar material as refractory tube 44.

[0055] This invention was tested in a 300A cell having configuration asshown in FIGS. 5 and 6. In FIG. 5, the cell was comprised of anodicliner 4, anodes 6 and cathodes 10. A molybdenum tube 44 was passedthrough openings 76 in the bottom of cathodes 10 (see FIG. 6) andinserted into alumina reservoir 34. Openings or slits 58 were providedadjacent cathode faces to receive molten aluminum deposited at thecathode during cell operation. Opening 74 in alumina reservoir 34 wasprovided with less than 0.25 mm clearance for tube 44. It was found thatif opening 74 was coated or sprayed with a material wettable withaluminum, e.g., molybdenum, a seal was facilitated to exclude bath. Theopenings 76 are shown in bottom of cathodes 10 in FIG. 6 which is across-sectional view along line C-C of FIG. 5. The cathodes werecomprised of TiB₂ and the anodes were comprised of Fe—Ni—Cu alloy. Alayer of bath 45 was provided in reservoir 34 to avoid oxidation ofmolten aluminum 62. The electrolyte in cell 4 consisted essentially ofNaF:AlF₃ eutectic, about 45 mol. % AlF₃ and had 6 wt. % excess aluminadispersed therein. The cell was operated for 4-6 hours at a temperatureof 760° C. and a current density of 100 amps. After operation, it wasfound that aluminum was collected in reservoir 34.

[0056] While reference herein has been made to TiB₂ cathodes, it will beunderstood that the cathodes can be comprised of any suitable materialthat is substantially inert to the molten aluminum such as zirconiumboride, molybdenum, titanium carbide and zirconium carbide.

[0057] The anode can be any non-consumable anode selected from cermet ormetal alloy anodes inert to electrolyte at operating temperatures. Thecermet is a mixture of metal such as copper and metal oxides and themetal alloy anode is substantially free of metal oxides. A preferredoxidation-resistant, non-consumable anode for use in the cell iscomprised of iron, nickel and copper, and containing about 1 to 50 wt. %Fe, 15 to 50 wt. % Ni, the remainder consisting essentially of copper. Afurther preferred oxidation-resistant, non-consumable anode consistsessentially of 1-30 wt. % Fe, 15-60 wt. % Ni and 25 to 70 wt. % Cu.Typical oxidation-resistant, non-consumable anodes can have compositionsin the range of 2 to 17 wt. % Fe, 25 to 48 wt. % Ni and 45 to 70 wt. %Cu.

[0058] The electrolytic cell can have an operating temperature less than900° C. and typically in the range of 660° C. (1220° F.) to about 800°C. (1472° F.). Typically, the cell can employ electrolytes comprised ofNaF+AlF₃ eutectics, KF+AlF₃ eutectic, and LiF. The electrolyte cancontain 6 to 26 wt. % NaF, 7 to 33 wt. % KF, 1 to 6 wt. % LiF and 60 to65 wt. % AlF₃. More broadly, the cell can use electrolytes that containone or more alkali metal fluorides and at least one metal fluoride,e.g., aluminum, fluoride, and use a combination of fluorides as long assuch baths or electrolytes operate at less than about 900° C. Forexample, the electrolyte can comprise NaF and AlF₃. That is, the bathcan comprise 62 to 53 mol. % NaF and 38 to 47 mol. % AlF₃.

[0059] As noted, thermal insulation 18 is provided around liner 4. Also,a lid 3 shown in FIG. 2 is provided having insulation sufficient toensure that the cell can be operated without a frozen crust and frozensidewalls.

[0060] The vertical anodes and cathodes in the cell are spaced toprovide an anode-cathode distance in the range of ¼ to 1 inch.Electrical insulative spacers 5 (FIG. 3) can be used to ensuremaintenance of the desired distance between the anode and cathode. Inaddition, bottom edge 54 of cathode 10 should be maintained at adistance of ¼ to 1 inch from bottom 38 of anode liner 4 in order toensure adequate current density and gas evolution on the bottom to keepalumina suspended.

[0061] In the present invention, the anodes and cathodes have a combinedactive surface ratio in the range of 0.75 to 1.25.

[0062] In the low temperature electrolytic cell of the invention,alumina has a lower solubility level than in conventional Hall-typecells operated at a much higher temperature. Thus, in the presentinvention, solubility of alumina ranges from about 2 wt. % to 5 wt. %,depending to some extent on the electrolyte and temperature used in thecell. Higher temperatures will result in higher solubility levels foralumina. To ensure against anode effect, an excess of alumina oversolubility may be maintained in the electrolyte. Thus, the cell can beoperated with a slurry of alumina (undissolved alumina) in theelectrolyte in the range of 0.2 to 30 wt. % with a preferred slurrycontaining undissolved alumina in the range of 5 to 10 wt. % alumina.The ranges provided herein include all the numbers within the range asif specifically set forth. Alumina can be added from hopper 70 (FIG. 2)to the space between electrodes and wall of sides 32 of liner 4. Thealumina is added in an amount such that the density of the slurry doesnot exceed 2.3 g/cm³, which is the density of molten aluminum.

[0063] During operation of the cell, oxygen is produced at the anodesurfaces and bubbles upwardly through electrolyte slurry 45 producingstirring in the cell. The stirring resulting from the evolution of gasbubbles provides dissolution of alumina in the electrolyte and aids inmaintaining saturation of dissolved alumina. The stirring also ensures aconstant supply of dissolved alumina to the anode surface. Further, thegas bubbles maintain undissolved alumina particles in suspension in thecell and prevents or inhibits alumina particles from settling to thebottom of the cell. Thus, it will be seen that the anodic linerimportantly contributes to evolution of gaseous bubbles to enhance theperformance of the cell, and thus suspended alumina particles aremaintained substantially uniformly distributed throughout theelectrolyte. Bayer-type alumina particles may be used and areapproximately 100 μm in diameter, but composed of an agglomeration ofsmaller particles. Ground alumina with 1 μm particles has been used inlaboratory tests.

[0064] Alumina useful in the cell can be any alumina that is comprisedof finely divided particles. Usually, the alumina has a particle size inthe range of about 1 to 100 μm with a preferred size being in the rangeof 1 to 10 μm.

[0065] In the present invention, the cell can be operated at a currentdensity in the range of 0.1 to 1.5 A/cm² while the electrolyte ismaintained at a temperature in the range of 660° to 800° C. A preferredcurrent density is in the range of about 0.4 to 0.6 A/cm² of anodesurface. The lower melting point of the bath (compared to the Hall cellbath which is above 950° C.) permits the use of lower cell temperatures,e.g., 730° to 800° C., which increases current efficiency in the celland reduces corrosion of the anodes and cathodes.

[0066] In another embodiment of the invention, the temperature of thecell may be controlled during operation of the cell or during reducedcurrent usage at peak power demand periods when the cost of electricityis high. Thus, referring to FIG. 1, there is shown means 80 for heatingcell 2, for example, during startup when no current is flowing or duringperiods of reduced operation such as reduced current flow during periodsof peak power usage. Means 80 is comprised of gas burner tubes 82 (shownin outline form) which extend underneath sides 38 of bottom 36 (seeFIGS. 2 and 7). In FIG. 1, means 80 for heating cell 2 is provided witha gas inlet 84 and an exhaust gas outlet 86. That is, when cell 2 isrequired to be heated, gas is introduced at 84 and burned with air inburners 82. The exhaust gases are removed at end 86 of the gas burnertube. By reference to FIG. 7, it will be seen that gas burner tubes 82are provided with openings 88 which emit gas for purposes of controlledburning with air. The flow of gas to burner tube 82 may be controlled orregulated using a gas control 98 well known in the gas industry. The gascontrol also determines the amount of heat being applied to the cell. Byreferences to FIG. 2, it will be noted that tubes 82 are provided withopenings 90 which permit air to flow underneath bottom 36 of cell 2 forpurposes of burning when the cell is being heated. As noted, exhaustgases are removed at exit 86 during heating. An air pump 94 may be usedto forcibly introduce air through air inlet 92 (FIG. 7) to the burnersand remove gases after burning.

[0067] The use of metal liner 4 permits the addition of heat to the cellas described.

[0068] The use of external heat applied through liner 4 in the mannerdescribed is useful in cell startup situations. For purposes of startup,electrolyte or bath is added to the cell in powdered form and placedbetween the anodes 6 and cathodes 10. Gas burner tube 82 is lighted andheat is transferred through metal liner 4 or bottom 36 to melt thepowdered electrolyte. As the electrolyte melts, additional electrolyteis added to bring the bath to operating level 46. Alumina is dissolvedin the electrolytic melt. Thereafter, electrical current is passedthrough the cell and the electrolyte to produce aluminum. Theelectrolytic production of aluminum generates heat and thus the gasheaters are turned off.

[0069] When the cell is operational, heat must be removed. The cell isdesigned so that heat lost therefrom during operation is less than theamount of heat generated by the electrochemical reaction within thecell. That is, during operation of the cell to produce aluminum, theheat loss through the sides, lid and bottom of the cell is less thanheat generated in the cell as a result of the electrochemical reaction,resulting in an accumulation of extra heat in the cell. The extra heatmust be removed at a controlled rate to maintain the cell at steadystate during operation. Thus, for purposes of the present invention, airis introduced to tube 92 using air pump 94. From tube 92, air isdirected underneath bottom 36 and exhausted through exit 86 to removeheat from the cell. As noted, air is circulated underneath the bottom ofthe cell using air pump 94 to control the rate of flow across the bottomof the cell to remove heat therefrom. Air pump 94 is controlled by anelectronic controller such as a programmable logic controller (PLC). ThePLC receives readings or signals from thermocouple 96 which monitors thetemperature of electrolyte 45. In the controller, the reading or signalfrom the thermocouple is compared to a set signal or reading. Inresponse to the comparison, the air flow rate can be increased,decreased, or maintained. That is, the PLC can be programmed to increaseor decrease the flow of air from air pump 94, depending on thetemperature of the electrolyte, and thus the electrolyte is maintainedin a controlled temperature range.

[0070] Thermocouple 96 may be protected from bath 45 by any suitableprotective sleeve which is resistant to attack by molten electrolytewhen the thermocouple is immersed in the bath. The protective sleeve maybe comprised of graphite or molybdenum, for example. If excess aluminais used in the bath to provide a slurry, a protective alumina sleeve maybe used. In another embodiment, the thermocouple may be placed in a wallsuch as a side wall to monitor the temperature of liner 4 to maintaintemperature control. In this embodiment, a protective sheath is notrequired. Thus, the temperature of cell 2 may be controlled in thismanner without the need for anode and cathode adjustments and thus thecell can be set for optimum production.

[0071] The electrolytic production of aluminum requires considerable useof electric power and thus, the cost of such power has a large impact onthe cost of producing aluminum metal. Accordingly, it is extremelyimportant to find the lowest cost electric power. For example, it isimportant to avoid the usage of electric power during peak power periodsbecause the cost of the power is much higher than off-peak power rates.Thus, it can be seen that there is great economic incentive to reducecell current during peak power periods. In the present invention, cellcurrent can be reduced or even stopped during peak power periods withoutconcern for freezing electrolyte and molten aluminum contained in thecell. When the electric current is reduced, heat can be added to thecell through bottom 36 using gas heater 82. That is, when the PLCdetects a sufficient drop in temperature of electrolyte 45 bythermocouple 96, it sends a signal to gas controller 98 to introducesufficient gas along with air to gas heater 82 to heat cell 2. Anelectronic ignition can be used to ignite the gas and the PLC can beused to continuously adjust the gas and air flow rates to maintain atemperature such that the electrolyte and aluminum contained in the cellremain in molten condition. At the end of the peak power period, fullcell current is used again, gas is turned off and the cell returned tonormal operation. In this way, the cell can be operated to effectivelyavoid daily peak power rates.

[0072] In another aspect of the invention, the effect of peak powerrates can be avoided in the low temperature cell in another way. Thatis, current flow to the cell can be reduced during peak power rates fromoperating level to a level sufficient to maintain the cell in moltencondition. Thus, voltage of the cell can be reduced from a normaloperating voltage of about 3.5 volts and heat must be added. At about3.1, e.g., 3.062 volts, zero heat is generated. The difference between3.5 and 3.1 volts, i.e., 0.4 volts, produces the designed heat lossthrough the insulation. Reducing the voltage to 2.3 volts stops thecurrent and metal production. Heat must be added to the cell using anoutside source, as described herein at a voltage and current below thedesign level. Thus, it will be appreciated that a combination of reducedvoltage and outside heat as described may be used and such iscontemplated. After the period of peak power rates, the flow is returnedto normal operating conditions.

[0073] The following example is further illustrative of the invention.

EXAMPLE

[0074] An apparatus was used comprising the liner for the 300A cell anda single molybdenum (Mo) cathode. The cathode was located beneath theelectrolyte and was a flat plate, ⅛″ (0.32 cm) thick, of rectangularcross section except at the bottom. The bottom edge was brought to apoint in the center of the cross section, with the bottom edges atangles of about 7 degrees from horizontal. Under the electrolyte, thiscathode plate measured 4″ (10.2 cm) across, 4″ (10.2 cm) high along eachoutside edge, and 4.25″ (10.8 cm) height in the center (at the point).These two sloped-bottom edges meeting at the point had attached to themMo tubing. The tubing outside diameter (OD) was ¼″ (0.64 cm), and theinside diameter (ID) was ⅛″ (0.32 cm). Each piece was about 2.01″ (5.1cm) long. This tubing was slotted over each length such that the bottomedges of the cathode each resided within the corresponding piece oftubing, with a clearance between the side of the cathode and the closestedges of tubing meeting the criteria of Eq. (2). The two pieces oftubing were butted together at the bottom point of the cathode, wherethey met. A hole was provided in one side of these tubes to allowconnection to another such tub of Mo of the same ID and OD, which passedfrom that face of the cathode perpendicularly to that face, and at anangle of about 15 degrees downward from horizontal. This served as theconveyance from the cathode to a collection chamber, and had a totallength of 2″ (5.1 cm).

[0075] The collection chamber comprised a length of closed-end roundbottom alumina tubing. The chamber was situated such that it was about1.5″ (3.8 cm) from the face of the cathode. Thus, about ½ inch of theconveyance tube resided within the walls and internal space of thistubing.

[0076] The alumina tubing had an ID of b 1{fraction (3/8)}″ (3.50 cm)and an OD of 1⅝″ (4.13 cm). The curvature for the closest end beganabout 11⅜″ (28.9 cm) from the open end, and the total length of thepiece as 12″ (30.5 cm). At a distance of about 11⅛″ (28.3 cm) from theopen end, a hole was centered in the tubing. This hole had a diameter ofabout {fraction (5/16)}″ (0.80 cm). On the alumina circumference of thishole, and on the outside of the tubing around the hole in a roughlycircular area of about 1″ (2.54 cm) in diameter, Mo was applied by aflame-spray method. The conveyance tube was then placed to enter thechamber through this Mo-coated hole. The distance between the holecoating and the outer surface of the conveyance tube met the conditionof Eq. 2. With this arrangement, the point of the cathode was about 1⅜″(3.50 cm) from the bottom of the anode liner of the cell while thebottom of the alumina tubing rested on the bottom of the anode liner,and the minimum distance from the bottom of the liner to any cathodemetal (in particular, the lowest point of the flame-sprayed Mo) wasabout ⅝″ (1.6 cm).

[0077] Because Mo oxidizes readily in air at elevated temperatures, theabove assembly was lowered into already-molten electrolyte prior to theelectrolysis test described below. The anode liner holding theelectrolyte, which was the only anode in this test, was of an investmentcast 10:15:15 Cu:Ni:Fe alloy.

[0078] The electrolyte was about 45 mol. % aluminum fluoride (AlF₃) and55 mol. % sodium fluoride (NaF). 3000 g were used at an operatingtemperature of about 760° C. The electrolyte was maintained as a slurrywith undissolved alumina, above saturation. The weight percent excessundissolved alumina was about 6%, and the alumina particle size wasnominally one micron. Electrolysis was conducted at 100 amperes for atotal of 5.1 hours in this test.

[0079] In this test, the cathode itself, conveyance tube andflame-sprayed Mo had been wetted with aluminum (Al) in a previous test.When the apparatus was inserted into the melt, the Al melted quickly anda seal performed. A heated stainless steel siphon tube connected througha valve to a vacuum was inserted into the collection chamber to a depthabout ½″ (1.27 cm) above the top of the hole in the chamber.

[0080] After about one hour of electrolysis at 100 amperes, a length oftungsten (W) wire was inserted into the chamber until it touched theclosed end at the bottom thereof. This was then pulled out andinspected; such procedure constituting a measurement of the depth ofboth Al and electrolyte in the chamber. The Al depth was determined tobe 1.8″ (4.6 cm), and the electrolyte layer above this appeared to bequite thin, about 0.04″ (0.1 cm). This depth represented more Al thanwould be produced in the one hour of electrolysis, and included Alpreviously present on the cathode assembly.

[0081] After another 1.38 hours of continued electrolysis, the Al depthwas measured again and found to be about 2.3″ (5.8 cm) deep. Theincrease in depth corresponds to an addition of about 12.2 ml of liquidAl, which was about 28 g at 760° C. This is about 61% of the totalamount of Al the electrolysis would produce during this time.

[0082] After an additional ¾ hour, the Al depth had climbed only another0.1″ (0.25 cm). At this point in the test, a vacuum was applied to thesiphon tube. Once the siphon was drawing only air, the depth wasmeasured in the collection vessel, and found to be 1.1″ (2.8 cm). Whenthe vacuum was subsequently removed, this level climbed an additional0.2″ (0.5 cm). Assuming that this siphon procedure collected 1.1″ (2.8cm) of Al depth, a total of about 61.6 g of Al was collected with thesiphon.

[0083] After these procedures, electrolysis was maintained for anadditional two hours. The depth at the end of this period was measuredto be only 1.5″ (3.8 cm). Thus, little Al was collected in the chamberafter the initial siphoning.

[0084] After the test, a total of 119.8 g of Al was recovered,representing a current efficiency of about 60% based on this recoveredmetal. Of the total recovered, about 62 g was found to have beencollected with the siphon. It was noted that the region that had beensprayed with Mo now had a significant amount of the intermetallicmaterial that forms at the interface of Al and Mo phases. This materialis mushy at temperature and does not flow readily. It is believed thatthe reason the Al depth ceased to climb in the collection chamber afterthe measurement taken 1.38 hours into the test is that the mushymaterial impeded the free flow of liquid Al into the chamber.

[0085] This test showed that (a) the principles of Eq. (2) function toform an effective seal between the chamber and the electrolyte, (b) theliquid Al formed on the cathode can be conveyed to a collection chamberdriven by the difference in hydrostatic head at the bottom of thecathode and in the chamber, and (c) liquid Al can be siphoned from sucha chamber once it has collected there.

[0086] While the invention has been described in terms of preferredembodiments, the claims appended hereto are intended to encompass otherembodiments which fall within the spirit of the invention.

What is claimed is:
 1. An improved method of producing aluminum in anelectrolytic cell containing alumina dissolved in an electrolyte, themethod comprising the steps of: (a) providing a molten salt electrolyteat a temperature less than 900° C. having alumina dissolved therein inan electrolytic cell having a liner for containing said electrolyte,said liner having a bottom and walls extending upwardly from saidbottom, said liner being substantially inert with respect to said moltenelectrolyte; (b) providing a plurality of non-consumable anodes andcathodes disposed in said electrolyte; (c) passing an electric currentthrough said anodes and through said electrolyte to said cathodes,depositing aluminum on said cathodes, and generating oxygen bubbles atthe anodes, said bubbles stirring said electrolyte; (d) periodicallyreducing electric current flow to said cell for extended periods; and(e) maintaining said electrolyte and aluminum in said cell in a moltencondition during said extended periods of reduced current flow byapplication of heat to said bottom for purposes of heating said cell. 2.The method in accordance with claim 1 including operating said cell tomaintain said electrolyte in a temperature range of about 660° to 800°C.
 3. The method in accordance with claim 1 including using anelectrolyte comprised of one or more alkali metal fluorides.
 4. Themethod in accordance with claim 1 including maintaining 0.2 to 30 wt. %undissolved alumina particles in said electrolyte to provide a slurrytherein.
 5. The method in accordance with claim 2 wherein undissolvedalumina has a particle size in the range of 1 to 100 μm.
 6. The methodin accordance with claim 1 wherein said anodes and anodic liner arecomprised of an Ni—Cu—Fe alloy.
 7. The method in accordance with claim 1including passing an electric current through said cell at a currentdensity in the range of 0.1 to 1.5 A/cm².
 8. The method in accordancewith claim 1 including using cathodes selected from the group consistingof titanium diboride, zirconium boride, titanium carbide and zirconiumcarbide.
 9. The method in accordance with claim 1 including providingsaid anodes and said cathodes substantially vertical in said electrolyteand arranging said anodes and said cathodes in alternating relationship.10. A method of efficiently operating a low temperature cell for theelectrolytic production of aluminum from alumina dissolved in a moltensalt electrolyte in a manner which is regulated to consume electricalpower in a more cost-effective basis, the method comprising the stepsof: (a) providing a molten salt electrolyte at a temperature less than900° C. having alumina dissolved therein in an electrolytic cell havinga metallic liner for containing said electrolyte, said liner having abottom and walls extending upwardly from said bottom, said liner beingsubstantially inert with respect to said molten electrolyte; (b)providing a plurality of non-consumable anodes disposed substantiallyvertically in said electrolyte and a plurality of cathodes disposedvertically in said electrolyte, said anodes and said cathodes arrangedin alternating relationship; (c) passing an electric current throughsaid anodes and through said electrolyte to said cathodes, depositingaluminum on said cathodes, and generating oxygen bubbles at the anodes,said bubbles stirring said electrolyte; (d) periodically reducingelectric current flow to said cell for extended periods; and (e)maintaining said electrolyte and aluminum in said cell in a moltencondition during said extended periods of reduced current flow byapplication of heat to said bottom for purposes of heating said cell.11. The method in accordance with claim 10 including operating said cellto maintain said electrolyte in a temperature range of about 660° to800° C.
 12. The method in accordance with claim 10 including using anelectrolyte comprised of one or more alkali metal fluorides.
 13. Themethod in accordance with claim 10 including maintaining 0.2 to 30 wt. %undissolved alumina particles in said electrolyte to provide a slurrytherein.
 14. The method in accordance with claim 11 wherein undissolvedalumina has a particle size in the range of 1 to 100 μm.
 15. The methodin accordance with claim 10 wherein said anodes and anodic liner arecomprised of an Ni—Cu—Fe alloy.
 16. The method in accordance with claim10 including passing an electric current through said cell at a currentdensity in the range of 0.1 to 1.5 A/cm².
 17. The method in accordancewith claim 10 including using cathodes selected from the groupconsisting of titanium diboride, zirconium boride, titanium carbide andzirconium carbide.
 18. An improved method of producing aluminum in anelectrolytic cell containing alumina dissolved in an electrolyte, themethod comprising the steps of: (a) providing a molten salt electrolyteat a temperature less than 900° C. having alumina dissolved therein inan electrolytic cell having a metallic liner for containing saidelectrolyte, said liner having a bottom having an outside surface andhaving walls extending upwardly from said bottom, said liner beingsubstantially inert with respect to said molten electrolyte; (b)providing a plurality of non-consumable anodes and cathodes disposed insaid electrolyte; (c) passing an electric current through said anodesand through said electrolyte to said cathodes, depositing aluminum onsaid cathodes, and generating oxygen bubbles at the anodes, said bubblesstirring said electrolyte; (d) removing heat from said cell through saidbottom of said liner by passing an air sweep from outside said cell oversaid outside surface of said bottom to remove heat from said bottom toprovide a heated air sweep; and (e) discharging said heated air sweep tothe atmosphere outside said cell thereby maintaining said cell at saidtemperature.
 19. The method in accordance with claim 18 includingoperating said cell to maintain said electrolyte in a temperature rangeof about 660° to 800° C.
 20. The method in accordance with claim 18including using an electrolyte comprised of one or more alkali metalfluorides.
 21. The method in accordance with claim 18 includingmaintaining 0.2 to 30 wt. % undissolved alumina particles in saidelectrolyte to provide a slurry therein.
 22. The method in accordancewith claim 19 wherein undissolved alumina has a particle size in therange of 1 to 100 μm.
 23. The method in accordance with claim 18 whereinsaid anodes are comprised of an Ni—Cu—Fe alloy.
 24. The method inaccordance with claim 18 including passing an electric current throughsaid cell at a current density in the range of 0.1 to 1.5 A/cm².
 25. Themethod in accordance with claim 18 including using cathodes selectedfrom the group consisting of titanium diboride, zirconium boride,titanium carbide and zirconium carbide.
 26. The method in accordancewith claim 18 including providing said anodes and said cathodessubstantially vertical in said electrolyte and arranging said anodes andsaid cathodes in alternating relationship.
 27. A method of efficientlyoperating a low temperature cell for the electrolytic production ofaluminum from alumina dissolved in a molten salt electrolyte in themethod comprising the steps of: (a) providing a molten salt electrolyteat a temperature less than 900° C. having alumina dissolved therein inan electrolytic cell having a liner for containing said electrolyte,said liner having a bottom having an outside surface and have wallsextending upwardly from said bottom, said liner being substantiallyinert with respect to said molten electrolyte; (b) providing a pluralityof non-consumable anodes disposed substantially vertically in saidelectrolyte and a plurality of cathodes disposed vertically in saidelectrolyte, said anodes and said cathodes arranged in alternatingrelationship; (c) passing an electric current through said anodes andthrough said electrolyte to said cathodes, depositing aluminum on saidcathodes, and generating oxygen bubbles at the anodes, said bubblesstirring said electrolyte; (d) removing heat from said cell through saidbottom of said liner by passing an air sweep over said outside surfaceof said bottom to provide a heated air sweep; (e) discharging saidheated air sweep outside said cell; (f) sensing the temperature of saidelectrolyte to provide a reading; (g) relaying said reading to acontroller; (h) in said controller, comparing said reading to a setreading to provide a comparison; and (i) in response to said comparison,increasing, decreasing or maintaining air flow rate in said air sweep tomaintain said cell at temperature.
 28. The method in accordance withclaim 27 including operating said cell to maintain said electrolyte in atemperature range of about 660° to 800° C.
 29. The method in accordancewith claim 27 including using an electrolyte comprised of one or morealkali metal fluorides.
 30. The method in accordance with claim 27including maintaining 0.2 to 30 wt. % undissolved alumina particles insaid electrolyte to provide a slurry therein.
 31. The method inaccordance with claim 28 wherein undissolved alumina has a particle sizein the range of 1 to 100 μm.
 32. The method in accordance with claim 27wherein said anodes and anodic liner are comprised of an Ni—Cu—Fe alloy.33. The method in accordance with claim 27 including passing an electriccurrent through said cell at a current density in the range of 0.1 to1.5 A/cm².
 34. The method in accordance with claim 27 including usingcathodes selected from the group consisting of titanium diboride,zirconium boride, titanium carbide and zirconium carbide.
 35. Animproved method for startup of a low temperature, electrolytic cell forproducing aluminum from alumina dissolved in an electrolyte at less than900° C., the method comprising the steps of: (a) providing anelectrolytic cell having a metal liner for containing electrolyte, saidliner having a bottom having an outside surface and having wallsextending upwardly from said bottom; (b) providing a plurality ofnon-consumable anodes and cathodes disposed in said electrolyte; (c)adding solid electrolyte and alumina to said cell; (d) placing at leastone heater adjacent said outside surface of said bottom; (e) adding heatto said bottom until said solid electrolyte is melted; and (f) when saidelectrolyte is in molten form, passing an electric current through saidanodes and through said electrolyte to said cathodes, thereby depositingaluminum at said cathodes and generating oxygen bubbles at the anodes.36. The method in accordance with claim 35 including adding heat untilsaid electrolyte is in a temperature range of 660° to 800° C.
 37. Themethod in accordance with claim 35 including using an electrolytecomprised of one or more alkali metal fluorides.
 38. The method inaccordance with claim 35 including maintaining 0.2 to 30 wt. %undissolved alumina particles in said electrolyte to provide a slurrytherein.
 39. The method in accordance with claim 2 wherein undissolvedalumina has a particle size in the range of 1 to 100 μm.
 40. The methodin accordance with claim 35 wherein said anodes and metal liner arecomprised of an Ni—Cu—Fe alloy.
 41. The method in accordance with claim35 including passing an electric current through said cell at a currentdensity in the range of 0.1 to 1.5 A/cm².
 42. The method in accordancewith claim 35 including using cathodes selected from the groupconsisting of titanium diboride, zirconium boride, titanium carbide andzirconium carbide.
 43. The method in accordance with claim 35 includingproviding said anodes and said cathodes substantially vertical in saidelectrolyte and arranging said anodes and said cathodes in alternatingrelationship.